This invention pertains to medical electrical pulse generators and electrical leads generally and more particularly to cardioversion and defibrilation leads and lead systems.
It has been known for many years that ventricular fibrillation, an often fatal arrhythmia, can be terminated by means of application of high energy electric current to the heart. Originally, this defibrillation current was applied to the patient by means of chest paddles in conjunction with a line powered, external defibrillator. While this method has been and most likely will continue to be the primary mode of defibrillation in the emergency room, it has been recognized that it is desirable to construct a totally implantable defibrillation system which is capable of detecting the onset of fibrillation and defibrillating the patient without the often fatal delay involved in getting the patient to an external defibrillation device.
It is known that by applying the electrical current directly to the heart, such as during open heart surgery, the amount of energy required to defibrillate the heart can be dramatically reduced. Typically, this type of defibrillation is accomplished with reduced size defibrillation paddles placed one each on the left and right ventricle of the heart. For example, in U.S. Pat. No. 2,985,172, issued to W. C. Jones, use of two paddle shaped mesh electrodes applied to the epicardium of the heart is disclosed.
An early attempt to produce an implantable electrode system for defibrillation of the heart is illustrated in U.S. Pat. No. 3,942,536 issued to Mirowski et al. In this electrode system, a single right ventricular endocardial lead is used, having one set of electrodes at its distal tip for location in the apex of the right ventricle and a second set of electrodes spaced from the set of electrodes on the distal tip a sufficient distance to place them in the superior vena cava. Other endocardial ventricular defibrillation lead systems are illustrated in U.S. Pat. No. 3,857,398 issued to Rubin and in U.S. Pat. No. 4,355,646 issued to Kallok.
In the recent past, it has been determined that the power required to defibrillate the human heart using a lead system such as described in the above patents, while significantly less than that required by the use of an external defibrillator, is still sufficiently large to make construction of a battery powered fully implantable defibrillator difficult. In addition, the relatively small surface area of the endocardial electrodes can result in extremely high current densities in the immediate vicinity of the electrodes, during application of the defibrillation pulse. This factor is important because the possibility of tissue damage increases as current density increases, and an endocardial defibrillation pulse is typically orders of magnitude greater than a typical cardiac pacing pulse.
In an attempt to create an improved defibrillation lead system, all epicardial systems have been proposed. One such is found in U.S. Pat. No. 4,030,509 issued to Heilman et al, which discloses an all epicardial system employing large surface area electrodes, one set to be applied at the apex of the heart, a second set to be applied to the atria of the heart. As an alternative, it is suggested that a superior vena cava electrode on an endocardial lead may be used in conjunction with a large electrode applied to the apex of the heart, typically referred to as an apical cup electrode.
Other large surface area electrodes for application to the human heart are disclosed in U.S. Pat. No. 4,291,707 issued to Heilman et al, which discloses electrodes fabricated of metallic mesh, sandwiched between two layers of chemically inert electrically insulative material. However, the electrodes disclosed in the Heilman applications suffer from the drawback that their surface area is essentially fixed, while, of course, the surface area of the heart to which they are sutured varies during contraction.
Recently, it has been proposed that rather than delivering electrical energy between electrodes located in the apex of the heart and electrodes located on or in the superior vena cava or atrium of the heart that a return to application of electrical energy transversely across the heart is desirable. For example, in published European patent application Publication No. 0 095 726 by the Purdue Research Foundation, it is proposed that four epicardial mesh electrodes be arranged orthogonally around the heart and that defibrillation be accomplished using two sequential orthogonal defibrillation pulses.